The present invention relates to a method of forming a joint at an interface between two sintered bodies comprising metallic oxides of specific crystal structure.
When employing sintered bodies in a device, such as an oxygen separation device, it is often mandatory to join the same securely or even to provide a gas-tight joint, said joint being required to withstand operation conditions of the device. Typical sintered bodies in the above devices are an ion transport membrane (an electrolyte), an interconnect, a support, ceramic tubes, seals and conduits, etc. Such sintered bodies are typically joined tube-to-tube, tube-to-flat-plate and flat-plate to flat-plate, respectively.
Any joint is likely to form the weakest point of the entire device. Weak points are critical in cases where the device is subjected to severe operation conditions such as high temperature, high pressure differences or highly oxidizing or reducing environments which are tolerated by the sintered bodies themselves. To provide a commercially viable device, the joint is thus likewise required to maintain mechanical integrity, compatibility with the sintered bodies and gas-tightness even when subjected to the operating conditions. Accordingly an ideal joint would possess comparable chemical and mechanical properties as the materials to be joined, especially comparable thermal cycling stability.
Up to now, joints between sintered bodies have been formed by using metallic brazes, nanocrystalline oxides, oxide-metal eutectics, glasses and ceramic-glass composites. See, e.g., S. D. Peteves et al., “The reactive route to ceramic joining: fabrication, interfacial chemistry and joint properties”, Acta mater. Vol. 46, No. 7, (1998), pp. 2407-2414; Y. lino, “Partial transient liquid-phase metals layer technique of ceramic metal bonding”, J. of Mat. Sci. Lett. 10, (1991), pp. 104-106; S. Serkowski, “Application of ceramic-metal eutectics for solid-state bonding between ceramics,” Int. Symp. Ceram. Mater. Compon. Engines, 4th (Roger Carlsson et al. eds.) (1992) pp. 348-355; M. Neuhauser et al.“Fugen von Technischen Keramiken mit Keramik-Grunfolien,” Ber. DGK, Vol. 72, No. 1-2, (1995) pp. 17-20; D. Seifert et al. “Verbind poroser mit dichtgesinterter Al2O3-Keramik durch Fugen mit keramischen Folien,” Ber. DGK, Vol. 73 No. 10 (1996) 585-589; and R. Chaim et al. “Joining of alumina ceramics using nanocrystalline tape cast interlayer,” J. of Materials Research, 15, (2000) pp.1724-1728.
Joining of sintered bodies using ceramic-metal eutectics has the disadvantage of requiring the use of a metal. Many metals oxidize in air at high temperatures and therefore require the use of special reducing atmospheres to prevent the formation of a metal oxide. The sintered bodies to be joined may not be stable in these reducing atmospheres, which would result in decomposition of the sintered bodies.
Joining of sintered bodies using nanocrystalline interlayers has the disadvantage of requiring very high pressures that could damage the parts to be joined due to creep or even fracture.
The use of brazes, i.e., metallic materials, or glasses, i.e., solid solutions of multicomponent metallic oxides, has the disadvantage of leaving behind an interfacial phase of the joint material with properties differing from, and in most cases inferior to, those of the materials being joined. For example, brazes leave behind a ductile metal, which at elevated temperatures can creep, be incompatible with the surrounding ceramic materials, or oxidize.
Similarly, glass joints may have significantly different thermal expansion coefficients compared with surrounding multicomponent metallic oxides having perovskitic or fluoritic structure, resulting in undesirable residual stresses following temperature changes. Glass joints will further soften and flow at temperatures above their respective glass transition temperature. Finally, glass joints can be chemically incompatible with a sintered body of perovskitic or fluoritic structure at elevated temperatures. In any case, due to the remaining material, the joint will inevitably be visually or microscopically detectable, its properties being determined by the material of the joint itself, not the bodies to be joined.
Rabin et al., “Reaction processing and properties of SiC-to-SiC joints.” Material. Res. Soc. Symp. Proc. 314, (1993), 197-203, Material Research Society, Pittsburgh, discloses another method of forming a joint, wherein SiC components can be joined by using a mixture of Si and C powders. The document is silent on joining oxides in general, and especially on joining of multicomponent metallic oxides having fluoritic or perovskitic structure.
Seifert et al. discloses a method to join alumina ceramics using ceramic joining foils of alumina-titania-calcia-magnesia. Other joining foils of alumina-titania-calcia-magnesia-silica and alumina-titania-manganese oxide-iron oxide-silica are also described. The joining temperature was greater than 100° C. lower than the sintering temperature of the alumina ceramics to be joined. These joining compositions formed a liquid phase upon heating to the joining temperature. After joining, the joint retained the composition of the joining foils and was compositionally different than the alumina bodies that were joined. This reference states that the joining compositions to be used are highly specific to the ceramics to be joined. This reference is silent on how to join multicomponent metallic oxides. It is specifically silent on how to join perovskitic multicomponent oxides.
Another method to join alumina ceramics, which is disclosed in Neuhauser et al., requires the use of ceramic foils made from a mixture of alumina, silica and other oxides. The presence of silica is undesirable since silica can be chemically or mechanically incompatible with the ceramics to be joined. In addition, this reference is also silent on how to join multicomponent metallic oxides.
A third method to join alumina parts using a (Al,Cr)2O3—Cr eutectic joining mixture is disclosed in Serkowski. To obtain the joint, special gas atmospheres to produce extremely low oxygen partial pressures were required to allow the joining mixture to melt. The requirement of these special gas atmospheres limits the ceramics with which the eutectic mixtures can be used. Many ceramics will not be stable under the low oxygen partial pressure conditions needed for the eutectics to melt. Also the eutectic joining mixtures will result in the joint material being chemically and mechanically dissimilar to the bodies to be joined. This will have a negative effect of the stability and integrity of the joint. In addition, this reference is silent on joining multicomponent metallic oxides.
A fourth method to join alumina is disclosed in Chaim et al. This method requires hot pressing the alumina parts to be joined under uniaxial pressures of 55-80 MPa at 1200-1300° C. This method has the alleged advantage that the joint material is chemically and mechanically identical to the parts to be joined. However, the high pressures necessary to produce the joint are undesirable since the high pressures can lead to fracture or creep of the ceramic parts to be joined. In addition, this reference is also silent on how to join multicomponent metallic oxides.
Another type of bonding has been developed which is the so-called transient liquid phase bonding (TLP). See, e.g., Y. Zou et al., “Modelling of transient liquid phase bonding”, Int. Mat. Rev. Vol. 40, No. 5, (1995), p.181, and I. Tuah-Poku et al., “Study of the Transient Liquid Phase Bonding, etc.”, Metallurgical Transactions A Vol. 19A, March 1988, p. 675. This process relies on the transient formation of a liquid phase depending on solute diffusion.
In many applications it is acceptable and oftentimes desirable to use a liquid phase or a transient liquid phase in order to join ceramic material. For example, see our prior U.S. Patent Application Publication Nos. 2004/0185236 A1 and 2004/0182306 A1 to Butt et al., which disclose liquid phase or transient liquid phase joining at below the sintering temperature. Both applications teach the use of low pressure and low joining pressures. They also teach the concept of high packing density and the ability to conform to the joining surfaces. These applications do not disclose or suggest that it is possible to obtain a joined material having uniform mechanical and thermal properties throughout the material, wherein the joining process is conducted without the use of a liquid phase.
Moreover, for some ceramics, the transient liquid phase produces undesirable second phases in the joint or in the adjacent ceramics to be joined. These second phases can result in joints with inferior mechanical properties. For example, when using a material with high chemical expansivity the use of a liquid phase may result in chemical gradients, which create stress upon thermal cycling.
One example where the transient liquid phase approach produces joints with inferior mechanical properties is the joining of LSCO (La1-xSrxCoO3-d) ceramics using a CuO—Ca2CuO3 eutectic joint material that produces a transient liquid phase containing Cu. After joining, a region with a high concentration of a cobalt oxide second phase, for example, has been observed when using Cu as a liquid forming additive. The phase change between CoO and Co3O4 in the second phase during temperature cycling from the joining temperature to room temperature introduces a tensile stress in the surrounding LSCO perovskite matrix and leads to cracking of the matrix. Providing a joining composition chemically identical to the material to be joined would solve this problem.
For other ceramics it is not possible to identify a transient liquid phase composition that produces a joint material that is chemically and mechanically compatible with the ceramics to be joined. The joining temperature should not exceed the sintering temperature of the material in order to limit grain growth. In many instances it is also desirable to join below the sintering temperature in order to limit creep deformation of the component being joined.
Therefore, the ability to join without a transient liquid phase at low pressures and low temperatures relative to the sintering temperatures is highly desirable and would be an improvement in the art. Low temperatures are defined as a temperature at least 100° C. below the sintering temperature. The sintering temperature is defined as the temperature required to reach greater than 95% of theoretical density. Low pressures are defined as pressures at the seal of less than 5 MPa and preferably less than 2 MPa.
It is therefore desired to provide a method of forming a joint between a first sintered body comprising a first multicomponent metallic oxide having a crystal structure of the perovskitic or fluoritic type and a second sintered body comprising a second multicomponent metallic oxide having a crystal structure of the same type as the first multicomponent metallic oxide, which method allows for formation of a joint that is chemically and mechanically compatible with the first and second sintered bodies. It is further desired that the formation of the joint does not leave behind a distinguishable interfacial phase. It is still further desired that the method should further allow for forming a compatible, refractory interfacial phase or joint, especially a joint exhibiting comparable thermal cycling stability.
All references cited herein are incorporated herein by reference in their entireties.